Process for determining the position of a radar target

ABSTRACT

A process is disclosed for determining the position and/or the displacement in time (trace) of a (radar) target surface by a radar installation. A predeterminable number M of the largest echo amplitudes is first determined for the target surface, as well as the corresponding resolution cells (distance and azimuth coordinates). By averaging the distance and azimuth coordinates of the selected resolution cells, coordinates are determined for a reference point that is stable in space in relation to the target surface. This reference point is used for locating and/or tracing objects in particular ships.

BACKGROUND OF THE INVENTION

The invention relates to a process for determining the position of aradar target extending over several resolution cells. More particularly,the invention relates to a position-determining process of the typewherein echo signals reflected from resolution cells are mixed in thebaseband so that an amplitude-modulated signal of the radar target isgenerated; in the baseband, at least for all resolution cells to beassociated with a radar target the amplitude values are determined; andthe position of the radar target is determined from the size of theamplitude values and/or their distribution in the area defined by theresolution cell.

The term "resolution cell" used in the present patent application isintended to cover the terms "radar resolution cell" as well as "arearesolution cell". A radar resolution cell identifies the spatialresolution of a radar installation. A radar resolution cell is adesign-dependent constant of the radar installation. In an arearesolution cell, several radar resolution cells are combined as needed.Therefore, an area resolution cell is considerably larger in its spatialextension than a radar resolution cell.

The invention relates in particular to the determination of the positionof a large-surface radar target such as, for example, an oceangoingfreight ship or passenger ship. Such radar targets comprise severalresolution cells, for example, a few hundred, when such a radar targetmoves past a radar set at a relatively small distance and/or when theradar set has a high spatial resolution. This condition exists, forexample, in the (radar) monitoring of harbor access routes and/orshipping routes. There, it is necessary, for example, to determine andto track the position of one or several ships as accurately as possiblewith a stationary radar set, which ships generally have littlemaneuverability because of their size, so that, for example, compliancewith a predetermined route can be checked and/or advance warning of apossible collision can be given.

Such target surfaces, for example, a ship having a length ofapproximately 200 m, due to their design, e.g., the superstructuresand/or the freight, may have a plurality of radar reflectors. A movingship also generates bow waves as well as stern waves (stern sea) whichalso reflect radar waves and thus change the extension of the actualradar target, the ship. A particularly disturbing effect on thedetermination of the target position results from reflections which aregenerated on the ship itself and which melt into the useful echoes.Furthermore, a radar installation may also create apparent radartargets, for example, due to the so-called minor lobes of the radarantenna, which apparent radar targets can also move along with theactual radar target, the ship.

If, for such a (radar) situation, the position of a (radar) targetsurface, for example, the ship, is now intended to be determinedaccurately solely on the basis of the radar information, for example, byway of the spatial resolution capacity possible per se of the radarinstallation of, for example, one tenth of the length of the ship, it isfirst necessary to find a characteristic (radar) reference point for thedesired radar target, the ship, and to then track the position of thereference point continuously in time.

In this context, one might consider selecting for this (radar) referencepoint the reflector with the largest (radar) echo amplitude. But thismethod fails in a disadvantageous manner if several equivalentreflectors and/or apparent reflectors are arranged so as to be closelyadjacent to one another.

For a large-surface radar target, which is comprised of several actualand/or apparent individual targets, one might consider selecting andtracking a surface center point or surface center of gravity of theradar target as a characteristic reference point by way of an averagingand/or integration method. A reference point determined in this mannercan also greatly change its position in a disadvantageous manner andthus simulate a change of position of the actual target, for example, ofthe ship. Such a case arises, for example, when the stern sea of a shipchanges considerably, e.g., as a result of a speed change of the ship.In such a case, there is a great change of the radar reflection to beassociated with the stern sea, which radar reflection changes the(radar) position of the (radar) surface target in a disturbing mannerbecause of the above-mentioned averaging and/or integral formation.Similar problems arise as a result of aspect-dependent reflections onthe ship itself.

SUMMARY OF THE INVENTION

It is the object of the invention to propose a position-determiningprocess of the generic type mentioned at the onset by means of which areliable determination of a reference point to be associated with theradar target becomes possible for a large-surface radar target.

This object is accomplished by a position-determining process which ischaracterized in that

in a predeterminable region of each associated resolution cell fromwhich at least one echo signal is received an amplitude value isdetermined which corresponds to the echo signal,

from the amplitude values of a resolution cell a maximum amplitude valueis determined which is associated with the resolution cell,

from the maximum amplitude values of all resolution cells the M largestamplitude values are selected, with M being a predeterminable, integer,positive number,

the coordinates (E, AZ) of the associated resolution cells aredetermined for the M largest amplitude values,

the associated mean values (E(F), AS(F)) are formed from the coordinates(E, AZ), and

the mean values (E(F), AZ(F)) form the coordinates of a reference pointwhich determines the position of the radar target.

A first advantage of the invention is that the determined (radar)reference point is stable relative to the (radar) shape of the (radar)target surface. The latter can therefore move without any disturbingjumps of the reference point, i.e., simulating fast apparent spatialchanges. Thus, it is assured that a reliable trace tracking of a targetbecomes possible.

A second advantage is that, for the above-mentioned larger ships, thereference point essentially corresponds to the actual center of the shipso that the determination of the position by way of the reference pointcorresponds in a reliable manner to the actual position which is alwaysindicated for the center of the ship.

Further advantages result from the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing an example of radar echo signals; and

FIG. 2 is a process flow chart for implementing the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is based on the evaluation of demodulated radar echosignals in the baseband, e.g., the video band. There, the echo signalsassociated with a radar target as well as with a resolution cell arerepresented as amplitude-modulated signals, i.e., a large amplitudevalue is allocated to a good (radar) reflector. It is advisable toselect the size of the resolution cells to be considerably smaller,e.g., ten times smaller, than the size of the radar target to beexpected, e.g. of a ship. This accomplishes a good (radar)representation of the radar target.

If a predeterminable area, which is composed of a plurality ofresolution cells, includes a large-surface radar target, e.g., a ship, areference point defining the large-surface radar target isadvantageously determined by means of the SGS method (SGS is the Germanacronym for Schwerpunkt-Varfahren mit Hilfe einer Geordneten Statistik:center of gravity method with the assistance of an ordered statistics).For a ship, this reference point advantageously corresponds to thecenter of the ship. In the SGS method, those resolution cells having theM largest amplitudes are first selected from the area, with M being apredeterminable, integer, positive number. This selection of the Mlargest amplitudes is possible in different ways. For example, theamplitudes can be selected from the entire area or only from a spatiallylimited partial area which, for example, has a strip-shaped form. Thelatter may comprise, for example, the radar representation of a shipincluding all reflections as well as the associated bow waves and sternwaves. In the selection of these M largest amplitudes, the allocation tothe respective resolution cell is maintained in each case.

The determination of the largest amplitudes preferably takes place inthat the distance range and the azimuth range of the radar monitoringarea are quantized for digital data processing. Each echo is completelycharacterized by amplitude, distance and azimuth. In each distancequantum, there exists a moving window detector for the targetrecognition criterion and a maximum window arranged azimuthally fordetermining the amplitude maximum in this distance quantum. If severalamplitudes in a distance quantum are at the same level, a definitemaximum amplitude does not exist. Then, the mean position of allidentical amplitudes is relevant. The maximum amplitude is allocated tothis mean position. For all maximum amplitudes determined in thismanner, the mean positions are formed and from these an amplituderanking, which is stored in a list, is formed by means of the orderedstatistics. Subsequently, the largest amplitude appearing in the list isselected.

On the basis of this (in absolute terms) largest amplitude value, thenext-smaller maximum amplitudes are selected, namely M-1piece. Thesewere also determined by way of the ordered statistics method.

This determination of the M largest amplitudes as well as of theassociated resolution cells of a target surface is carried out with eachrotation of the radar antenna or a comparable pivot process of theantenna lobe.

If the antenna lobe leaves the target surface F during the pivotprocess, a reference point having the distance coordinate E(F) as wellas the azimuth coordinate AZ(F) is formed for the target surfaceaccording to the formulas

    E(F)=S(E)/M and AZ(F)=S(AZ)/M.

Here,

S(E) is the sum of all distances, measured from the ship or other radarantenna, of the resolution cells which are associated with the Mselected amplitudes;

=S(AZ) is the sum of all azimuth values which are associated with the Mselected resolution cells; and

=M is the number of the resolution cells=number of the selectedamplitude values.

A reference point determined in this manner is stable in an advantageousmanner, i.e., is associated with the (radar) shape (shape of a targetsurface on a radar screen) of a target surface and thus largelyindependent of the time (number of pivot processes of the antenna lobe).Such a reference point can therefore be used reliably for determiningthe position as well as for the tracking of a target surface. It isparticularly advantageous that, for a larger ship, a reference pointwhich was determined in this manner corresponds to the center of theship. Therewith reliable tracking becomes possible.

FIG. 1 shows a two-dimensional representation (distance E and azimuthAZ) of an example of a baseband video signal, digitized in thehexadecimal code (zero to F), for an ocean region. The distance (orrange) E is plotted in arbitrary distance units and the azimuth AZ isplotted in degrees. The corners of zeroamplitude resolution cells aremarked by four adjacent grid points, and hexadecimal amplitude valuesare shown in resolution cells where the video signal has an amplitudegreater than zero (that is, 1 to F). In the example of FIG. 1, eachradar echo is completely characterized by a hexadecimal amplitude value,a distance value, and an azimuth value.

FIG. 1 shows that a large surface target (a ship F) must be presentbetween the distance values 410 and 440. With the present invention, theM highest amplitude values are selected. This can advantageously be donewith ordered statistics. The highest amplitude values are all locatedwithin a frame shown by a dashed line, which can be selected from thebeginning. The position of the center of the ship F is determined withthe above formulas. The thus-obtained position of the center of ship Fis marked in FIG. 1.

In FIG. 2, radar echo signals from the ship F are received in step 10and the baseband video signal is generated in step 12. This is convertedto digital in step 14. For each resolution cell, the maximum amplitudevalue, the distance value, and the azimuth value are then determined instep 16. Then the M highest amplitude values are selected in step 18 andtheir associated distance and azimuth coordinates E and AZ aredetermined. The average distance and azimuth values are found from theselected values in step 20 and used to designate the center of the shipF in step 22.

The process described is not limited to the application of monitoringship traffic but can be applied in many different ways to further(radar) targets. For example, the size of a resolution cell can beselected according to the size of the target surface and/or the accuracyof the location of the reference point to be determined. It isadvisable, for example, to select the number M as a variable that can beset at a valve which is determined empirically. In high-resolution radarinstallations, M is selected to be larger than in low-resolution radarinstallations. Furthermore, M is a function of the absolute level of theamplitude (values) of the echo signals. For example, for a targetsurface to which small amplitudes are allocated, the number M should besmaller than for a target surface of identical size which, however,generates larger echo amplitudes, i.e., represents a better radarreflector.

The invention is therefore not limited to the examples described but itcan be applied analogously to all (radar) target surfaces wherein areference point is necessary for determining the position and/or themovement (trace).

What we claim is:
 1. A process for determining the position of a radartarget extending over several resolution cells, comprising the stepsof:(a) mixing echo signals reflected from resolution cells in thebaseband so that an amplitude-modulated signal of the radar target isgenerated; (b) in the baseband, determining amplitude values for atleast all associated resolution cells that are associated with the radartarget; and (c) determining the position of the radar target, step (c)including(c-1) in a predeterminable region of each associated resolutioncell from which at least one echo signal is received, determiningamplitude values which correspond to the echo signal, (c-2) from theamplitude values of each associated resolution cell, determining amaximum amplitude value for the respective associated resolution cell,(c-3) selecting the M largest amplitude values from the maximumamplitude values of all associated resolution cells, with M being apredeterminable, integer, positive number, (c-4) determining distanceand azimuth coordinates of the M associated resolution cells having theM largest amplitude values, and (c-5) finding mean distance and azimuthvalues from the distance and azimuth coordinates, the mean valuesproviding coordinates of a reference point which indicates the positionof the radar target.
 2. A method according to claim 1, wherein at leastthe M largest amplitude values are determined by an ordered statisticsmethod.
 3. A method according to claim 1, wherein the maximum amplitudevalue of each associated resolution cell is determined by an integrationmethod wherein all amplitudes of the respective resolution cell areadded.
 4. A method according to claim 1, wherein each resolution cellhas a size, and wherein the number M is selected as a function of atleast one of the size of the resolution cells and the size of themaximum amplitude values.
 5. A method according to claim 1, wherein eachresolution cell has a size and the target has a size, and wherein thesize of the resolution cells is selected to be considerably smaller thanthat of the radar target.
 6. A method according to claim 1, wherein theradar target is a ship and the reference point is set to be the centerof the ship.
 7. A position-determining process, comprising the stepsof:(a) receiving echo signals reflected by a radar target which extendsover a plurality of resolution cells; (b) determining a subset of theresolution cells from which the reflected echo signals are stronger; (c)finding an azimuth coordinate and a distance coordinate for each of theresolution cells in the subset; and (d) finding a mean azimuth valuefrom the azimuth coordinates and a mean distance value from the distancecoordinates, the mean azimuth and distance values serving as coordinatesof a reference point which indicates the position of the radar target.